This invention relates to a fail-safe logic circuit and more particularly to a pulsating logic AND gate for providing fail-safe operation.
In the design of fail-safe logic circuitry, it is recognized that solid state components are equally likely to fail in the ON state as in the OFF state. Not only can semi-conductor junctions electrically open circuit or short circuit with almost equal probability, but they can also "lock up" even though no catastrophic failure occurs. To minimize the impact of the above types of failures, two principles are important in fail-safe logic element design; (a) a true logic state must be equated to an "energy present" condition, and (b) inverted functions such as NOT, NOR and NAND should be avoided.
The AND gate circuit of this invention observes the foregoing principles to make its application in fail-safe digital systems such as oil and gas combustion controllers particularly attractive.